Since hydrogen-oxygen fuel cells can retrieve energy more efficiently, compared with internal combustion engines, the studies have been actively under way currently. Here, hydrogen, a fuel for fuel cell, is synthesized by steam reforming natural gases, methanol, and the like, and by the subsequent carbon monoxide shift reaction. Since carbon monoxide, a poisoning substance to fuel-cell electrodes, is contained in a trace amount in the obtained hydrogen, the removal of the impurity, carbon monoxide, in the reformed hydrogen has been required (10-50 ppm). As for a method of removing carbon monoxide, a method of removing it by oxidizing it, a method of separating hydrogen by means of membrane reactor, a method of removing it by hydrogenating it to methane, and so forth, are available.
Among the methods of removing carbon monoxide, one which becomes the main stream is a method of adding oxygen into reformed gases and removing it as carbon dioxide by oxidation. In this method, oxygen is added in order to remove carbon monoxide efficiently. Accordingly, the oxidation of hydrogen develops simultaneously with the oxidation of carbon monoxide so that not only hydrogen has been consumed wastefully but also it has become difficult to control the heat-generation by means of combustion reaction and to control the adding oxygen amount.
On the other hand, in the case of employing a membrane reactor, pressurized heat application is needed when collecting hydrogen with a membrane so that the complication of system is inevitable. Moreover, since hydrogen cannot be collected by 100%, the direct loss of hydrogen has taken place as well.
Since the removal method by means of reaction (methanation reaction) which hydrogenates carbon monoxide, included in a trace amount in hydrogen, is such that, contrary to the oxidation removal method, it is not necessary to newly add oxygen, and the like, a possibility of-simplifying the system is available.
As for a conventional removal method of carbon monoxide by means of methanation reaction, there is disclosed a removal method of trace-amount oxidized carbons in a hydrogen-rich gas, removal method in which oxidized carbons are reacted with hydrogen using a cobalt oxide catalyst at a methanation-reactor inlet temperature of about 120-250° C. (Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) No. 62-36,004). And, it is possible to name a removal method of carbon monoxide in a hydrogen-containing gas, which methanates carbon monoxide, making use of a catalyst in which a ruthenium compound and an alkali metal compound and/or an alkaline-earth metal compound are loaded on a refractory inorganic oxide support (Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) No. 2002-68,707). Moreover, it is possible to name a removal method by means of carbon monoxide methanation using a carbon monoxide removing catalyst in a hydrogen-containing gas, carbon monoxide removing catalyst which is completed by drying and reducing, without carrying out calcination, after loading a nitrate of ruthenium on a refractory inorganic oxide support (Patent Literature No. 3: Japanese Unexamined Patent Publication No. 2002-66,321).
Moreover, there is a description on a possibility of achieving a carbon monoxide methanation catalyst in which a Co metal and an Ru metal are loaded on an inorganic support including an oxide of Zr. In the examples, a catalyst in which an Ru metal is loaded on zirconia, an oxide of Zr, and a catalyst in which a cobalt metal is loaded on silica are disclosed separately (Patent Publication No. 4: Japanese Unexamined Patent Publication (KOKAI) No. 2004-97,859).
Further, there is disclosed a technology of removing carbon monoxide based on a general carbon monoxide selective oxidation method (Patent Publication No. 5: Patent Publication No. 3,593,358).